The present invention relates to a fuel cell. More specifically, the present invention relates to a fuel cell that generates electricity through progress of an oxidation-reduction reaction using an enzyme as a catalyst, the fuel cell achieving high output or the like through a simple fuel supply, and to a power supply device and an electronic apparatus using the fuel cell.
In recent years, fuel cells (hereinafter referred to as biofuel cells) in which a microbe or an oxidation-reduction enzyme that functions as a catalyst is immobilized on at least one of the anode and the cathode have received attention as next-generation fuel cells with high capacity and safety. This is because, in biological metabolism in a living body, high-efficiency energy conversion is performed at room temperature and thus high-efficiency power generation with high safety can be achieved; and by using an enzyme having substrate specificity, electrons can be efficiently extracted from a fuel such as glucose or ethanol with which a reaction cannot be easily caused using a typical industrial catalyst.
For example, in aspiration of microbes or cells, saccharides, proteins, fat, and the like are converted into electric energy by reducing nicotinamide adenine dinucleotide (hereinafter referred to as “NAD+”) to reduced nicotinamide adenine dinucleotide (hereinafter referred to as “NADH”) in a process of producing carbon dioxide through a glycolytic pathway and a tricarboxylic acid cycle.
Furthermore, in photosynthesis, by absorbing light energy, nicotinamide adenine dinucleotide phosphate (hereinafter referred to as “NADP+”) is reduced to reduced nicotinamide adenine dinucleotide phosphate (hereinafter referred to as “NADPH”), and thus the light energy is converted into electric energy.
As described above, in biological metabolism, since chemical energy including nutrients such as saccharides, fat, and proteins and light energy is converted into electric energy, fuel cells using such conversion have been developed.
For example, Japanese Unexamined Patent Application Publication No. 2000-133297 discloses a power generation method and a cell that use microbes or cells and an electrode used for the power generation method and the cell, the electrode being obtained by immobilizing an electron mediator. Furthermore, Japanese Unexamined Patent Application Publication No. 2001-208719 discloses an enzyme electrode obtained by immobilizing an oxidation-reduction enzyme in or on a polydialkylsiloxane film formed on a substrate electrode. Japanese Unexamined Patent Application Publication No. 2004-71559 discloses a fuel cell having a mechanism with which NADH is produced from alcohols or saccharides using NAD+-dependent dehydrogenase.
Herein, FIG. 9 is a diagram showing a reaction scheme of a biofuel cell. In the biofuel cell using glucose as a fuel and shown in FIG. 9, an oxidation reaction of glucose occurs at the anode and a reduction reaction of oxygen (O2) in the air occurs at the cathode. In addition, at the anode, electrons migrate in the order of glucose, glucose dehydrogenase, nicotinamide adenine dinucleotide (NAD+), diaphorase, a mediator, and an electrode (carbon).
Such power generation in biofuel cells have been conventionally performed by supplying a fuel in an aqueous solution state into an electrode.
For example, in Japanese Unexamined Patent Application Publication No. 2000-133297, a fuel is supplied by filling an anode cell with 0.1 M phosphate buffer solution (pH 7) containing 10 μM glucose (refer to paragraph 0040 of Japanese Unexamined Patent Application Publication No. 2000-133297). Furthermore, in Japanese Unexamined Patent Application Publication No. 2001-208719, power generation is performed by inserting an electrode coated with a polydimethylsiloxane film into 0.1 mol/L phosphate buffer solution (pH 7) (refer to paragraph 0013 of Japanese Unexamined Patent Application Publication No. 2001-208719). Furthermore, in Japanese Unexamined Patent Application Publication No. 2004-71559, power generation is performed by adding 1 M methanol or the like to 3 mL of 0.1 M tris-hydrochloric acid buffer solution (pH 7.0, I.S.=0.3) (refer to paragraph 0066 of Japanese Unexamined Patent Application Publication No. 2004-71559).
In an existing biofuel cell, since a fuel is supplied into an electrode in an aqueous solution state, a fuel solution with high concentration is not smoothly supplied into the electrode. Thus, even if an electrode designed so as to have a porous structure is used to increase the supply efficiency of the fuel, a reaction does not sufficiently proceed at the inner surface of the electrode, which poses a problem in that an output is decreased.
In addition, the occurrence of a phenomenon in which an enzyme, a microbe, a coenzyme, an electron-transferring mediator, or the like immobilized on an electrode leaches out into a fuel solution causes a problem in that an output is decreased.
Accordingly, a principal object of the present invention is to provide a fuel cell that can achieve high output or the like by a simple fuel-supplying method.